Thread: 3-jaw chuck question

It shouldnt give you any great concern,the lathe dosent go fast enough for it to be a worry,the missing piece would only weigh a couple of ounces.
If you are concerned just make a plug to fit in the space left by the missing part and scotch dowel it useing a grub screw.

Thanks Pipeclay, anyway, I tried it out with a length of silver steel and it turns out that is seems to be way off centre (i tried two different pieces and both wobble badly) so I've gone back to my 4 jaw independent. Any idea why it might be so off centre?

have you checked to see if the jaws are inserted in the right order if one is in the wrong order it will make itbe off centre. there should be some numbers stamped on the jaws when you turn the scroll i think its called (the bit that makes the jaws move in and out) each jaw has to be inserted in the right order ie number one as the scroll enters slot 1 then 2 as it enters slot 2 etc

First, how old is this chuck ?

Second, how much did this chuck cost you ?

A 3 jaw chuck was designed to grab something quickly and then you TURN the bit to accurate, try to make sure you can do this in one operation or take the 3 jaw of and fit the 4 jaw.

Very few 3 jaw chucks will be accurate over the whole range of the scroll, that's from very smallest piece, to the largest piece that the chuck can handle. Even brand new ones will not be totally accurate over the whole range, now, if the chuck was a cheapy, you know why it was so cheap.

If you paid a grand or more for it, then take it back and get what you paid for, some of the expensive ones are VERY accurate over the whole range.

Do you see a pattern appearing here, the more you pay, the better the item will be.

So yours has a wobble when you shove something into it, eh, re-read of the above.

Now the reasons for why this is occuring to your chuck are many, the scroll wears, the teeth that connect with the scroll wear, some DH has misused/mistreated the chuck, the jaw gripping surfaces wear/squashed or something has been spat out of the chuck at some time. Or it was a cheapy in the first place.
If the jaws on your chuck are in the correct places and you still reckon that they have excess wobble, you can trim them up with a boring bar quite easily. You have to have pressure on the jaws as you do it, or it will be even worse than what you started with. What you have to do is get something that can be placed right down the back of the teeth, like a large nut and tighten the jaws onto it, then very carefully start to take very light cuts over the surface of the 3 jaws, do not hit the nut doing this. The jaws can be removed and that little bit at the back of them, that the nut was hanging onto, can now be ground away from the new surface you have just put onto the jaws.

Now about here, some bright spark is going to chime in and say that the jaws will only be accurate at the particular spot, OK, they are 100 % right, but, the jaws will still be a bloody sight better over the whole range, than what they were before you did this.

Another alternative, go buy some brand new jaws, that's if you can find the brand of your chuck and even if they are available. Seeing as how you posted this in the Hercus section, well you wont have a hope of new jaws if the chuck is an original fitting. Might have to check if the scroll is any good as well, new jaws and old scroll equals run-out again.

So two choices, new chuck or rework what you got, good luck with it.

NO, make that three choices, if you really want accurate, then go buy a collet system that will work on your lathe, only downside to that is, the collet system might be worth more than what the lathe is.

NO, make that four choices, go buy another lathe, it might even be the bearing in the headstock in the lathe you got, causing this problem.


Mark

tanii51, yeah the numbers align.

Mark, thanks for the the run-down. It came with the lathe so I have no idea how old or how much. Headstock bearings are fine I assume since when I put dead centres in spindle bore and tailstock and and turn the lathe on, the dead centre in the spindle bore remians dead in line with the dead centre in the tailstock.
So I'm figuring it's a chuck problem. Think I'll stick with the 4-jaw independent.

Cheers,

DavidG